The present invention relates to a method of growing an InGaAsP crystal on an InP substrate.
A semiconductor laser of the type using an InP substrate and an InGaAsP active layer has recently been found to have excellent characteristics as a light source for optical fiber communications. The lasing wavelength of a semiconductor laser of the type described may be freely chosen within the range of 1.1-1.65 microns, a small loss wavelength range of optical fibers, by suitably selecting the composition ratio of InGaAsP. However, a detailed analysis of the lasing wavelengths of double-hetero structure semiconductor lasers which use the above-mentioned material shows that they undesirably oscillate in a plurality of axial modes at the same time or that, if oscillating in a single axial mode, they change to another axial mode with the lapse of time. It follows that where such a semiconductor laser oscillating at a wavelength near 1.5 microns (a wavelength providing a minimum optical fiber loss) is used, the pulse width of a pulse signal is increased while the signal propagates through the optical fiber, because the wavelength dispersion of the optical fiber is substantial at that wavelength. The increase in pulse width prevents high-speed communications.
An approach heretofore proposed to solve the problem discussed above is a distributed feedback (DFB) semiconductor laser. As shown in FIG. 1, the DFB laser has an InP substrate 11 and an InGaAsP guiding layer 12 adjoining each other along an interface 15 which has periodic corrugation. An InGaAsP active layer 13 which emits a laser beam is deposited on the guiding layer 12. Deposited on the active layer 13 are a cladding layer 14 for confining carriers and light. The layered structure shown in FIG. 1 is a basic structure and various modifications have been proposed. Typical of these modifications are the formation of another guiding layer between the InGaAsP active layer 12 and the InP cladding layer 14, the positioning of such a layered structure upside down, the depositing of a contact layer on the InP cladding layer 14, and the localization of the periodic corrugation of the interface 15. In the basic structure shown in FIG. 1, among the various light components propagating through the guiding layer 12, that one which satisfies Bragg's condition, which is determined by the period of the corrugation, is reflected and only a single axial mode which satisfies the condition can selectively lase.
The problematic situation with the manufacture of such a DFB laser is that a sufficient depth cannot be afforded for the periodic corrugation and an effort to form a sufficiently deep corrugation results in a limited emission efficiency.
In detail, the periodic corrugation is formed by an ordinary photoetching technique. This process is capable of forming a periodic corrugation which is as deep as 1,500 .ANG. without any problem. This is followed by epitaxial growth of the InGaAsP guiding layer 12 on the so corrugated InP substrate, and it has been found that this step shallows the corrugation. It has also been found that where the epitaxial growth is effected in such a manner as to preserve the corrugation, the resulting InGaAsP active layer is quite poor in crystal quality and, therefore, in emission efficiency.
Presumably, the decrease in the depth of the corrugation is accounted for by the occurrence of volatilization while the temperature is elevated to 600.degree.-700.degree. C. which is essential for epitaxial growth. Particularly, the projections or ridges of the substrate are volatilized and deposited in the recesses which alternate with the projections.
With liquid phase epitaxial growth, a method has been proposed which prevents the volatilization of the projections of the substrate by covering the top of the InP substrate with an InP wafer to set up an atmosphere of phosphorus vapor, as disclosed in K. Sakai et al. "1.5 microns Range InGaAsP/InP Distributed Feedback Lasers", IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-18, No. 8, 1982, pp. 1272-1278. There has also been proposed a method which adds phosphine in order to set up a sufficiently high vapor pressure, as described in N. Nagai et al. "Prevention of Surface Corrugation Thermal Deformation for InGaAsP/InP DFB Lasers", JAPANESE JOURNAL OF APPLIED PHYSICS, VOL. 22, No. 5, 1983, pp. L291-L283. These methods made it possible to form desired corrugations with hardly any decrease in depth so long as the temperature is not higher than 620.degree. C. 620.degree. C., however, is lower than the optimum temperature for epitaxial growth. Since the optimum temperature is 700.degree. C. for vapor phase epitaxy (VPE) and 650.degree. C. for liquid phase epitaxy (LPE), the epitaxial growth at a temperature on the order of 620.degree. C. lowers the crystallizability of the InGaAsP active layer and, thereby, the emission efficiency.
Another proposal heretofore made uses a GaAs wafer in place of the InP wafer, as taught in J. Kinoshita el al. "Preserving InP Surface Corrugations for 1.3 microns GaInAsP/InP DFB Lasers from Thermal Deformation during LPE Process", ELECTRONICS LETTERS VOL. 19, No. 6, p. 215. In accordance with this method, the vapor of arsenic (As), instead of the phosphorus vapor, protects the corrugated substrate. This method allows the temperature to be elevated up to about 620.degree. C. without reducing the corrugation depth. Nevertheless, upon elevation of the temperature beyond 620.degree. C., the substrate surface becomes susceptive to thermal damage due to the absence of phosphorus vapor; moreover, the available amount of As vapor is insufficient. This method, like the above discussed one, tends to reduce the corrugation depth and offers only limited crystalizability of the InGaAsP active layer and, thereby, only a limited emission efficiency. Further, Mito, one of the inventors of the present invention, found that the use of a GaAsP wafer as a protective substrate is effective (Japanese Patent Application No. 58-116341/1983). However, even the protective GaAsP wafer cannot set up a sufficient phosphorus and arsenic vapor pressure in a temperature range of 650.degree. to 700.degree. C., which is suitable for epitaxial growth, resulting in thermal damage to the surface 15 of the corrugated substrate which reduces the corrugation depth as previously mentioned.